1. G. B. Airy:Mathematical Tracts (Cambridge, 1831),p. 301, (ibid., 1858), p. 295.
2. B. Bleaney andR. P. Penrose:Proc. Roy. Soc, A89, 158 (1947);Proc. Phys. Soc,59, 418 (1947).
3. J. G. Powles:Nature,161, 25 (1948).
4. W. Gordy:Rev. Mod. Phys.,20, 668 (1948).